An organic light emitting diode (LED) (both polymer based organic LEDs and small molecule organic LEDs) consists of a stack of organic layers sandwiched between an anode and a cathode. The organic stack contains both the light-emitting layer or layers in case of stacking and one or more charge carrier and/or charge injecting layers. When a voltage is applied across the organic stack, light is produced.
In order to prevent degradation of the device by interaction with the ambient atmosphere, the device needs encapsulation. Water vapor is considered the harmful component. Even with an encapsulating cover a water absorbing and/or moisture absorbing getter material is inserted in the cavity between the cover and the substrate to remove water penetrating through the glue that is used as an edge sealant for the cover.
In order to exploit the potential of organic LEDs in commercial products, reliable operation of the devices is a prerequisite. Though the intrinsic lifetime is sufficiently long to allow several applications of organic LEDs, both as display and as light source, the occurrence of early failures by electrical shorts limits the practical lifetime of devices based on this technology.
The layered structure of an organic LED comprises individual thin layers, that are produced by processes such as, for example, vacuum vapor deposition or spin casting. In such coating processes, the presence of particles, such as dust particles for example, leads to layer defects. The dimensions of such particles are usually appreciably larger than the thicknesses of the individual layers. The size and shape of the layer defects depend on the position and geometry of the particle and on the point in time from which the particle was present on the growing layered structured during the production of the thin layers. Such layer defects may lead to the formation of electrical shorts between the anode and the cathode.
Electrical shorts between the cathode and the anode may be present in the device directly after processing or may be are formed during operation of the device. The former results in a decrease of the yield of production of the devices; the latter is a reliability issue in the application of the device.
The international application WO 2006/027736 discloses an electroluminescent arrangement having a layered structure with an organic luminescent layer between an anode and a cathode. A dielectric layer having an oxygen concentration of more than 2 ppm is provided for complete wetting of the layered structures to prevent electrical flashovers. Thereby, in the regions of layer defects, unintended conductive bridges between the cathode and anode are oxidized and hence their conductivity is reduced. In this way the risk of flashovers is reduced.
However, the content of oxygen in the dielectric layer is rather limited, and such a layer possibly interacts with the performance of the device. Thus, there is a continuing need for a device optimized with respect to efficacy and intrinsic lifetime and a remedy against shorts without concession to these parameters.